This relates generally to imaging systems, and more particularly to output gain measurement and calibration methods for charge-coupled device (CCD) imaging systems having multiple outputs.
Electronic devices such as cellular telephones, cameras, and computers often include imaging systems that include digital image sensors for capturing images. Image sensors may be formed having a two-dimensional array of image pixels that contain photodiodes that convert incident photons (light) into electrical signals. Electronic devices often include displays for displaying captured image data.
Conventional interline CCD imagers are provided with multiple photodiodes that are formed below a pinning layer. In a conventional imager, the photodiodes are typically n-type doped regions in a semiconductor substrate. The pinning layer formed over the photodiodes is usually a p-type doped layer. The pinning layer formed over the photodiodes is conventionally coupled to ground and serves as a ground for the photodiode. The potential of the photodiode remains constant as long as the voltage provided at the pinning layer is constant, and there is no net global current flow throughout the device.
Light incident on the imager results in the accumulation of photo-generated electrons in the n-type photodiode region. Some of these photo-generated electrons are read out into a vertical CCD (VCCD) by applying a read-out voltage (sometimes referred to as the “third-level voltage”) to a transfer gate that is formed over the VCCD and a region between the photodiode and the VCCD.
The “third-level voltage” conventionally used in the readout of photo-generated charges from photodiodes to the VCCD is usually a large voltage greater than 7 V.
The photo-generated electrons are then read out from each VCCD to a horizontal CCD (HCCD), where they are then transferred to an associated output. In some conventional CCD image sensors, an electron multiplier CCD (EMCCD) is incorporated into the HCCD path in order to increase magnitude of signal levels at the output so that they are in a readable range. In order to measure the gain provided by an EMCCD, conventional image sensors use a controlled light source that results in a known amount of charge being generated at each photodiode of the image sensor. This known amount of charge is then read out through both EMCCD outputs and non-EMCCD outputs so that an EMCCD gain measurement can be obtained. The controlled light source used in this method is often built into the system housing the imager. However, this method is undesirable for video capture operations, as it requires the video stream to be interrupted so that a light can be flashed to measure gain.
Some conventional CCD imagers use content in a captured image to measure EMCCD gain, but this method cannot be relied upon if the captured image only generates small signals in the CCD imager (e.g., in low light conditions).
Some conventional CCD imagers rely on defective pixels having high levels of dark current to measure EMCCD gain, but this method requires pixel defects to operate and is not applicable to CCD imagers having few or no defective pixels.
Some conventional CCD imagers rely on changes in EMCCD voltage or temperature to calculate EMCCD gain, but this method is not desirable because changing EMCCD voltage or temperature will alter the appearance of a captured image.
Some conventional CCD imagers rely on injecting charge directly into the entrance of an EMCCD in order to inject a known signal size for gain calibration. However, obtaining a ratio of signals with and without gain using such a method requires altering the EMCCD voltage, which is undesirable as it will alter the appearance of a captured image. This method is further undesirable as it does not provide a means for matching the outputs of more than one EMCCD.
Accordingly, what is needed is a technique that allows for the measurement and calibration of the output gains of multiple EMCCDs within a single CCD imager, while avoiding the disadvantages associated with the above-noted conventional techniques.